1. Field of the Invention
The present invention relates to a focus detection apparatus of an imaging apparatus, and particularly to a technique for enabling focus detection in a wider range.
2. Description of the Related Art
With a widespread use of imaging apparatuses represented by digital cameras and video cameras, demands for higher quality and downsizing of the imaging apparatuses are more and more increasing. In particular, there are demands for wider field of view and downsizing of a focus detection apparatus that detects the focusing state of an imaging lens of an imaging apparatus. A focus detection apparatus that employs the Through-The-Lens (TTL) phase difference method forms a plurality of optical images with a plurality of light fluxes each of which has passed through a different one of a plurality of divided regions of an exit pupil of an imaging lens, and then detects the focusing state of the imaging lens based on a relative positional relationship between the plurality of optical images.
In the above-described focus detection apparatus, a photoelectric conversion element for detecting information related to the focusing state detects light in a wide range of an imaging region. This enables focus detection over a wide range. The size of an optical image on the photoelectric conversion element is determined by the size of a viewing field mask for limiting a light flux entering a focus detection optical system, and the magnification of a secondary imaging lens for forming an optical image on the photoelectric conversion element. In a focus detection optical system having a large viewing field mask and a small secondary image forming magnification, the photoelectric conversion element can detect light in a wide range of the imaging region. However, an increase in the size of an optical element for guiding a light flux in a wide range to the photoelectric conversion element in the focus detection optical system or in the size of the housing of the focus detection optical system may result in an increase in the size of the focus detection apparatus. Further, a decrease in the magnification of the secondary imaging lens degrades the sensitivity of the focus detection apparatus to changes in a relative positional relationship between object images on the photoelectric conversion element plane with respect to defocusing. Thus, the focus detection accuracy decreases. For downsizing of a focus detection apparatus, a method for shortening the distance between a pair of optical images formed on a photoelectric conversion element is used (see Japanese Patent Application Laid-Open No. 2006-322970). However, if adjacent optical images overlap each other, the adjacent optical images become noise, resulting in degraded focus detection accuracy.
As described above, three elements “wide field of view”, “high accuracy”, and “small size” are in a trade-off relationship in principle. A focus detection apparatus having a balance among the three elements is demanded.
A focus detection apparatus discussed in Japanese Patent Application Laid-Open No. 2006-322970 achieves both wider field of view and downsizing. More specifically, the shape of the viewing field mask is designed so that a region where optical images overlap each other is not formed on a photoelectric conversion element, and further an imaging region is not provided in a region where optical images are close to each other. However, shortening the distance between optical images degrades the sensitivity of the focus detection apparatus to changes in a relative positional relationship between object images on the photoelectric conversion element plane with respect to defocusing. This may lead to degradation in detection accuracy.
The present invention is directed to a focus detection apparatus capable of performing focus detection in a wide field of view while avoiding detection accuracy degradation and size increase.